Middleweight monster doesn’t fit usual categories

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An artist's impression shows an intermediate-mass black hole lighting a nebula that is 100 light-years wide in the dwarf irregular galaxy Holmberg II. With a mass 25 to 40 times that of the sun, this black hole may be a remnant from the first generation of stars in the universe.

Astronomers have found what appears to be a black hole 25 to 40 times the mass of our sun, a weight class not previously known to exist.

Black holes can't be seen, because any light that enters them is trapped. So to find black holes, scientists look for intense radiation from around them as well as their gravitational effects on nearby gas and stars.

Black holes come in two distinct varieties, scientists know. A stellar black hole results from the collapse of a single, massive star and is typically a few times the mass of the sun. Supermassive black holes anchor the centers of many galaxies and can harbor millions or billions of solar masses.

"There's a big gap there," said Philip Kaaret of the Harvard-Smithsonian Center for Astrophysics. Kaaret thinks he's found one that helps fill the gap.

Yet several attempts to identify middleweights, those suspected of weighing hundreds or thousands of times as much as the sun, have not fully panned out. Some astronomers think middleweights might have been very important in the early universe, serving as an intermediate stage in the development of a stellar black hole into one of supermassive proportions. If so, there ought to be a few that didn't fully evolve and are still around as middleweights.

Scientists do not know whether supermassive black holes build in such a step-by-step process, or if perhaps they result from a sudden collapse of material or the merging of galaxies.

Elusive X-ray source
The new study re-examined a fuzzy cloud of material called a nebula, 10 million light-years away and inside a "dwarf irregular galaxy" named Holmberg II. The nebula, known for some time, is illuminated by X-rays from a source whose identity had been elusive.

The observations pinpointed the X-ray source at the center of the nebula and measured the X-ray output at about a million times all the radiation emitted by the sun. The X-rays are likely generated by a black hole consuming matter from a young, massive companion star, according to Kaaret, who led the study. He added that it is brighter than the surroundings of any other black hole in the Milky Way except the supermassive one in the center.

Every four years, the black hole candidate consumes an amount of material equal to the entire Earth. On the way in, the material is accelerated to a significant fraction of light-speed and superheated, giving off the X-rays. Black holes are known to be sloppy eaters in this sense.

There is only one other nebula known to be similarly lit by a powerful X-ray source.

"Astronomers always get excited about new things, and this nebula is certainly something new," Kaaret said.

Other suspects
Kaaret and his colleagues consider the black hole to fit into the intermediate-mass category. A handful of other middleweights have been suspected but not confirmed.

"It's not easy to explain how intermediate-mass black holes form," he said. "Since we only have a few examples to study, every new find is important."

The observations were made with the Hubble Space Telescope, the Chandra X-ray Observatory and the XMM-Newton spacecraft. The results were presented last week at the 204th meeting of the American Astronomical Society in Denver and will be published in the Monthly Notices of the Royal Astronomical Society.